These sites, freelancer, Odesk, people per hour, are something worthwhile if you're based in a developing country and you charge peanuts. As a UK designer, you just can't compete on price. And the majority of clients on these sites come to them because they're outsourcing what's been outsourced to them. So they'll never pay you anything close to even min wage. I've worked on one of them for years and I can safely say, that the only ones who pay fairly are UK based ppl who are legitimate businesses. The rest are looking for cheap, at the "best" quality and will only bleed you dry, as the person in your case is doing.
And the power lies with them not the seller. In the event of a dispute, the site will charge you a fee if they have to intervene, and the client gets final say on whether the revision was to an adequate standard, if they say it wasn't you don't get what's in the escrow. They also drop your rating, (the freelancer site)..
Take a moment to look at where the "top sellers" are based.. It will be India, and Eastern Europe.. Atleast in design.. And it's a myth that you can raise prices later, by starting out low. The people who use these sites come to them because they don't wish to pay market rates.. Or in their words "You're not a Real company" lol